The last one I tried uploading that didn't work (but had previously) was taken on my Samsung S6, 3.95mb, .jpeg resolution 5312x2988. Previously the site resized it, but recently it keeps crashing. I ended up resorting to editing (cropping) the photo on my phone to reduce it to 2.17mb at 2002x1992 which seemed to upload.
(As said I had posted the original previously without issue)