lionel - bonjour
Capture sounds like the Mac App 'Grab' - whose native filetype is TIFF. This SiteWare does not 'like' TIFFs that much either... here''s what happens to a 720K sample TIFF

Same image, Saved AS from Preview.App as a 104K PNG file

The SiteWare used to UPLOAD the file but only displayed a LINK that, when clicked, displayed the image in the "Viewer" PopUpPane. Apparently the fix for that was to stop supporting TIFFs altogether. 😉
Unless your App "Capture" has some very special annotation features, I would simply use OS X built-in ScreenCapture features.
⌘ Command + ⇧ Shift > Press ' 3 ' > release all three = entire display, menu and all
⌘ Command + ⇧ Shift > Press ' 4 ' > release all three = selection tool
* Both of the above save files to the Desktop
** You can capture to the Clipboard instead by ⌃ Control + ⌘ Command + ⇧ Shift > Press ' 3 '(or 4) > release all four > Paste - like this

There are other more robust Apps that folks here use, both System wide and In-Browser add-ons