As you may have noticed by now, the forum is quite deserted, not many people visit here.
The best way to get attention is to make a branch of the graphics repository, and add your sprites to it.
If that fails for you, you may want to opt for adding the graphics itself to an issue in that project.
If that fails for you, you can try posting the files here, and making an issue with a link to the forum post.
As MarkL said, you can try the IRC if you have problems, but people are not always available (or are busy and cannot react immediately), so you may have to wait a while. Depending on your location that may or may not work nicely.
Anyway, I am not a developer for CorsixTH, but I think the first aim is to replace the current sprites, in particular more or less 1-to-1, so you should keep the same number sprites, and same use, as we have now. (Hopefully a real developer can confirm this for you soon.)
That means that if the original sprites have 3 parts for the text-speech bubble, you should also make 3 sprites (I don't know what it uses, so your current solution may be correct.)
Secondly, new sprites will be added in the SDL2 branch, which uses full colour (32bpp RGBA) sprites. Instead of 8bpp images that you have now, you should make 32bpp images. You thus also don't need to limit yourself to the current palette (although if you go for retro-looks, it may be useful to do so). It would be useful if you make everything 'outside' the sprite fully transparent. The rendering code skips the transparent parts, so it displays the background. Partial transparency is also allowed, although I don't know why you'd want to use it.
In all, it looks to me that you on the right path. Afaik nobody else is working on UI graphics, so they will come in handy.