Who even has a computer anymore... that's annoying.
There are photo processing app for tablets as well as for computers.
So you can do the same thing with your I-pad .
While you are using a tablet, the forum is runnin on a mainframe computer, quite a big & powerful one , so you are still using a computer.
The problem is the same though.
You take a photo on the tablet & the tablet sees the file as being 1 Mb but when the server sees it the file appears to be 20 Mb which is beyond the size limit.
Remember your tablet has just your photos on it and for many people there is not enough space so they either rent some on :the cloud" or buy massive storage devices.
The forum has thousands of members all posting photos so the storage space becaome massive.
Then there is the processing time to retreive that photo , format the page and send it down the wire, over the air however to your tablet / phone .
Now all of these steps have a maximum allowable time before the system says "this ain't working " and tosses up an error message or if it is beyond the capacity of the processing device, frezes.