I did put the custom images from another hud into steam\steamApps\common\team fortress 2\tf\custom\my_custom_stuff\materials\console. Currenty it stays the same blue
You need to rename your custom images to either background_upward or background_upward_widescreen, depending on if your screen is 4:3 or 16:9, 16:10, etc. and rename the vtf you replaced to something else.
I did put the custom images from another hud into steam\steamApps\common\team fortress 2\tf\custom\my_custom_stuff\materials\console. Currenty it stays the same blue