The plugin which caused the conflict was "Juiz Smart Mobile Admin", if you want to look at that. Other users have reported problems with that plugin, so I think the bug is more likely to be caused by Juiz Smart Mobile Admin, not your plugin.
Thanks very much for the tip about clicking, it works well now! However, I wonder if there is a better way to do this. Your plugin will work well if a website has different thumbnail sizes with the same ratio. However, if a website has two or three thumbnail sizes with different ratios, you have to press "deselect all" after every thumbnail. Maybe in a future version, you could change the functions, so that (for example) "click" automatically deselects the current thumbnail and selects the new thumbnail, and "shift + click" selects both the current and the new thumbnail.