Bookmark this site, and use it to upload your photos to IMGUR.
http://photoposting.is-great.net
Don’t worry about setting the height and width of your picture, as this site will automatically scale it down to the recommended size for this forum (400x600) if it’s too big.
I use this from all of my devices, iPhone, iPad, and windows PC.
After you upload your picture, copy and paste the entire link it provides you into the “Quick Reply” or “Advanced Post Form” on the forum. See what I’ve highlighted below.
From a mobile device, it’s of course better to upload images with a small file size, vs one that’s several MB’s. On Apple devices, they will give you the option of choosing the image size to upload, and “medium” is usually just fine. Not sure what androids let you do there.
:):)
