Squoosh it before uploading.

I had this issue too at the start. The image just seems to be too big. So it needs to be compressed a bit. Most of the time I just message it to myself and upload the then newly required file.

Image is to big, its a instance issue, some instances don't allow uploads at all and most impose size restrictions, try using a hoster outside of lemmy (postimages.org, catbox.moe etc.)