I had the uninstall issue as well. I figured it out so here you go.
Im assuming astro is what u are using as a file manager
First off dont "delete " the APK file from your sd card!
if you did reconnect to your computer and re add the apk to your phone. (you dont need to re install the file it is still installed you just deleted the APK file image from your phone.)
open astro/file manager , go to the apk skin file you want to delete. ( for example I put my Skins in the SD card Download folder [ /mnt/sdcard/download ] )
Just like when you installed it click the file (as mcaspe1 said dont long press just press and release)
you should see Application File and two buttons :
Browse file and
Open app manager
click Open app manager
then you should see an uninstall button. click it and it will ask for a confirmation that you want to remove the skin. hit yes and it will remove it.
if the apk image is still in the folder just delete it.
hope this helps
PS these skins are sick! I have almost all of them on my phone now.
Skins by gadget! -- for 2.1 ROMs Updated 9/06
PPS Im not rooted and running the stock ota GB and sense 2.1