I have just done some tests on my Android (Samsung S6) phone.
The basic issue is that one cannot upload multiple images in one go.
I did the following in Chrome but Firefox is very similar.
The Upload Image button pops up Choose Files and this pops up Documents. As with Firefox, one can upload one file at a time. Pressing Choose Files for the second
time just hangs with “Processing… please wait”.
If at that point I press Submit I get “Upload failed: error” and there is an OK button. Pressing that does sumbit the posting and shows that the original (first) image was uploaded correctly.
If however I now re-open the posting (with the Edit button) it opens with the cursor being positioned just after the previous image, and pressing Image Upload etc does work for the next image.
So one can upload multiple images quite easily; you just have to Submit after each one and the re-open it with Edit, then use the Image Upload button again. You have the standard 2 hour edit window.
I have just tested this with five files, and it works the same way with Firefox.
There may be an app which supports checkboxes in this context and then multiple images could possibly be uploaded in one go, but I haven’t seen one. And in any case the order might end up random-ish, which is usually no good (this happens on a PC with Windows Explorer; the proper image viewers like ACDSEE do upload in exactly the displayed order).
I haven’t tested multiple image file upload with IOS but the above solution should in any case work.