What's wrong with uploading to your forum? It's not hard from a phone (I've done it many times and I know some of my user do so too). If space it an issue then set the Maximum Attachment Image Dimensions to something like 1024 x 1024 (what I have set) to keep image file sizes small.
Having said that, setting the dimensions I suggested will typically produce JPEGs of 100-200Kb. That'll get you 5000-10000 JPEGs stored in 1Gb of space which still leaves you space for the XenForo files and other stuff. Setting smaller dimensions e.g. 800 x 800 would allow you even more photo storage in 1Gb.