Assuming you've clicked the insert image button, if there is one on the mobile platform (wouldn't know), you can find a site that your able to upload pictures to, just use that hosting site, like say imageshack or tinypic, then you can use tags on the forums to insert the image directly into your post using
Code:
[COLOR=#ff0000][img][/COLOR][COLOR=#0000ff]http://imageshack[/COLOR].com/[COLOR=#0000cd]picture.jpg[/COLOR][COLOR=#ff0000][/img][/COLOR]
the red tags are the opening and closing of image insert tags, just need url to image in middle, if you can upload it somewhere.
That would be an alternative to uploading directly to RIU, but I'm not sure what your platform looks like, or what glitches/changes ect are on other platforms like mobile phones.