As has been discussed many times, pictures to be uploaded to UKW as attachments have to be less than a set size limit. At present, it's 256kb. That's much smaller than most cameras or phones will produce, where images can easily be several megabytes.
Reducing file size can sometimes be a frustrating experience as you go through successive changes to image size, trying to find the point at which the file is small enough.
I've used IrfanView
for many years, as a programme for viewing, editing and arranging images. It's a deservedly popular bit of free software with many millions of downloads. (Donations are welcomed.)
I recently learned that if you use the 32-bit version and install the plug-ins pack, you can choose a target file size
to which your images should be shrunk. It seems to do a very good job of retaining as much image quality as possible while getting the file small enough to be acceptable.
There is a powerful batch mode which lets you select a group of photos, reduce their file size and rename them, all in one go.
The hompage at http://www.irfanview.com/
has links to download sites for the various versions and the plugins - the one that does the clever file resizing is snappily named "RIOT" meaning "Radical Image Optimization Tool".
The software is available for all versions of Windows but not for other operating systems.