I can't help you with the TT fender thingy since I don't have one, but can help you with the Mac not doing what you want. (I have however been screwed by muddy roads packing up my front wheel to the point where driving wasn't possible, and could have used one. Once.)
When you can't drap and drop a photo from a given website:
To capture a portion of the desktop, press Command-Shift-4. A cross-hair cursor will appear where your pointer was and you can click and drag to select the area you wish to capture. When you release the mouse button, the screen shot will be automatically saved as a PNG file on your desktop.
Then upload to display as you wish.