Sending a cute reaction is one of those things that seems to have no rules and has quite a lot of them. A GIF that lands perfectly in one app arrives as a broken link in another, a sticker that looks right in a message looks wrong in a comment, and some of them do not send at all depending on where you got them.
Here is what goes where, and why.
The formats are not interchangeable
Four different things get treated as the same, and they behave differently.
A GIF is an animated image file. It plays automatically in most places, loops, and carries no sound. It is the oldest format here and the most widely supported.
A sticker is a still or animated image with a transparent background, sent as its own object rather than as a message attachment. On iPhone, stickers can be placed on top of messages and photos rather than only sent alongside them.
A Live Photo is a still plus a short paired video. It is an iOS format and it does not survive being sent through most non-Apple apps, which convert it to a flat image.
A short video is a video. It carries sound, it does not loop by default, and it is the heaviest of the four.
The confusion is mostly between the first two, and the practical difference is transparency: a sticker sits on top of things, a GIF sits in a box.
Where each one belongs
Messages and iMessage. Stickers are the native currency here. They can be dragged onto a specific message, they stack, and they are part of the conversation rather than an attachment to it. GIFs work fine and read as more generic.
Group chats on other platforms. GIFs, almost always, because sticker support varies and a GIF is understood everywhere.
Comments and replies on social platforms. Depends entirely on the platform, and many accept neither. Where they do, the built-in picker is more reliable than pasting from elsewhere.
Email. Neither, generally. A GIF in an email is heavy and often blocked, and stickers do not exist as a concept there.
Why a GIF sometimes arrives as a still
The most common frustration in this whole area, and it has a dull explanation. Many apps and platforms recompress images on upload, and some convert animated files to a single frame in the process, particularly when the file is large. It is not about the app you sent it from, it is about what the receiving platform does on the way in. The practical fix is to send from the platform's own picker where one exists, since those serve a format the platform has already agreed to handle. Where there is no picker, a smaller file survives the trip far more often than a large one.
Making your own
Stickers from your own photos are the underused option. On iPhone, open a photo in Photos, touch and hold the subject to lift it from the background, and add it as a sticker. It becomes available in the keyboard's sticker drawer everywhere stickers work.
It works best on a clear subject against a distinguishable background, which is the same requirement the lock screen depth effect has, for the same reason.
For a GIF, the practical route is a short screen recording or a video trimmed and converted. Keep it under a couple of seconds. Long GIFs are large files, and large files are the ones that arrive as stills.
Reading the room, briefly
The format question is the easy half. The harder half is that a reaction image carries tone, and the tone it carries is not the same everywhere.
In a close group chat, a well-chosen reaction does more than a sentence and everyone understands the register. In a work conversation, the same image reads as a decision to be informal, which is fine in some workplaces and conspicuous in others.
The reliable approach is the same one that works for emoji: match rather than lead. Use roughly what the other people in the conversation are using. If nobody has sent one, you are the person introducing them, and that is a choice rather than a default.
One more thing worth knowing: a reaction image ends a thread more often than it continues one. That is useful when you want to close a conversation warmly, and unhelpful when you were hoping for a reply. If you need an answer, send words.
The size question
Worth knowing because it explains most failures.
An animated file's size grows with its length, its dimensions and the amount of movement in it. A three-second full-screen GIF can be several times the size of a one-second one, and it is the large ones that get converted, rejected or delivered slowly.
If something you send keeps failing, the first thing to try is a shorter or smaller version of the same thing rather than a different app.
Quick answers
Why do my stickers not appear in some apps?
Sticker support depends on the app implementing it. Where it is missing, the sticker usually falls back to being sent as an image.
Can I send a Live Photo to an Android phone?
It will arrive as a still image. The paired video half does not survive the conversion.
Do GIFs have sound?
No. If something animated arrived with sound, it was a video rather than a GIF.
Why is my own sticker's edge rough?
The cutout follows what the phone could separate from the background. A subject against a busy or similarly coloured background gives it less to work with.
Where to get it
Kawaii Keyboard & Wallpapers keeps GIFs, stickers and cute text in one keyboard, so the picker is the same wherever you are typing rather than different in every app.
If something will not send, make it shorter before you make it different. That fixes most of them.