Thanks! I now see me question was useless... Sorry... I did read "load" not "upload". When I tested uploading a larger image file (2.44 MiB) I got a HTTP timeout error.
I agree that these upload errors are probably due to large filesize. We should find out whether there is a maximum file size on Lemmy in general, whether it's instance specific or if it's actually a problem with Jerboa.
Edit: Looks like the root cause is not Jerboa but Lemmy: