Someone pointed out to me the other day it can be adjusted by the instance you're uploading to and it's incredibly small by default.
Where are you trying to upload the image? Cuz another thing I wonder may be happening is if you're trying to upload to Lemmy.world from your Lemmy.ml account since Lemmy.World defederated from Lemmy.ml it might just block anything you try to upload. Unless they reversed that? You're able to post this here so idk.