I'm not sure I follow. Can you post a screenshot please of what you are trying to set?

Selecting any of the available header images does not apply as the selection is stuck on / (from memory, deleted install)

As you can see, the select border is on / and the chosen image (furthermost to right) is not actually selected. Most of the header images don't even get a thin border like the one chosen.
admin-config&type=category&id=THEME

(Click to enlarge)

Ah gotcha, thank you for clarifying. I'll take a look.

I managed to select one with Beta4, but it seems it's fighting with the original.

EDIT: Managed to replace the background image by selecting N/A so now it's just 1 image but the image is still indented.

(Click to enlarge)

It seems to only work on fixed width and not fluid width. So I'll fix that.

Also the default Composr logo has its own background which clashes with that background. I don't even know if I have a transparent Composr +1 logo, but I'll see if I can find one and apply it.

Additionally, the borders you speak of... something is wrong, and if you click on one of the headers after having already clicked on it once, you don't get a border anymore. I'll see if I can fix that too. This seems to be a problem with the field type, not the field, as other image selection fields also have the same problem.

...and the help button for the classic header image selection is also broken. Lots of broke happening here.

Automated response: Misc theme issues

This hotfix fixes the following:

- Header images not working on fluid width (it wasn't supposed to, but I made it so with a caution it may not look the best)
- Default Composr +1 logo had a background which conflicted with the header background if selected; now the default logo is transparent
- Radio list with theme images wouldn't show the dotted selected border anymore after first selection
- Help icon on theme image radio lists were broken

A hotfix (a TAR of files to upload) has been uploaded to this issue. Only apply this hotfix if you absolutely need it and cannot wait until the next release of Composr (releases are more reliable and strictly tested). As of Composr version 11, the recommended way to apply a hotfix is by following the same steps as an upgrade (https://baseurl/upgrader.php, use the hotfix on the step “Transfer across new/updated files”). The upgrader will automatically skip files belonging to addons you do not have installed or that are newer on disk than in the hotfix. Otherwise, you can manually extract and replace these files (do not replace if your on-disk file is newer than the one in the hotfix). Always take backups of your site or at least files you are replacing before applying a hotfix. Not sure how to extract TAR files to your Windows computer? Try 7-zip (http://www.7-zip.org/).