I have uploaded quite a few images to posts and it seems to work ok. I think you are correct and that there is a size limit or limits. Limits may depend on file type.
There will be some nice tools for bulk re-scaling images for web use. Bound to be free ones I imagine. Make sure you keep copies of the originals.
2555 x 2332
1.43MB jpeg
seems ok
2988 x 5312
2.30MB jpeg
fails as you describe
Jupiter and her moons, 2555 x 2332, 1.43MB jpeg - gets auto scaled by pprune for display.
