Imgur is good. After you upload a photo, right click on it, and go 'open image in new tab'. Then you can just paste that url in the 'paste a file url' box here, to get a higher resolution image.
People can then right click on the image in bikeforums and go 'open image in new tab' and see the higher resolution.

https://www.bricelam.net/ImageResizer/
is a handy tool for resizing your photos and compressing them a little, it can do batches just from the context menu (right click on them - resize image)