I am not familiar with Windows, but since you asked for solutions for all OSes, I have an applescript solution for Mac OS X that I have tested by copying a picture on this website and executing the script.
This applescript assumes the image is on the clipboard in TIFF format (might have to test to see if this is what comes out of Excel.) It creates the file from the clipboard, saves it to a temporary directory, then pastes the path into a specified field on the frontmost page in Safari.
So, you would copy the image, switch to your safari page, and run the script. (From the script menu, make it into a service and assign a shortcut, or use FastScripts to assign a shortcut to the applescript.)
The script will have to be adjusted to find the proper field on your form.
repeat with i in clipboard info
if TIFF picture is in i then
-- grab the picture from the clipboard, set up a filename based on date
set tp to the clipboard as TIFF picture
set dt to current date
set dtstr to (time of dt as string) & ".tiff"
set pt to ((path to temporary items from user domain as string) & dtstr)
set tf to open for access file pt with write permission
-- save the file
write tp to tf
close access tf
close access tf
-- put the path into the proper field in the web Browser
tell application "Safari"
-- currently inserts into Answer textarea of this superuser.com page for testing
-- ie. make sure you've clicked "add answer" first
set myJS to "document.getElementById('wmd-input').value = '" & pt & "'"
-- document 1 is frontmost
Edit: Things to consider:
- I do nothing with the path, default delimiter is a colon. You may want the POSIX path.
- Excel supports applescript, and has a
copy picture command. It may be feasible to do this in one step. Select picture, run script, script copies, saves, opens web page and fills out the form.