You need to reduce the file size. I don't know how you are trying to upload, so can't help much. I take pics with my iPhone, but email them to myself at reduced resolution (about 250 kb) and it works pretty well, makes a good sized pic for the forum.
Ok. I don't use Tapatalk and was uploading from a file on my phone that was 1.79 MB. Says it's still "too large" even after I emailed it to myself. Odd.