Alpha channel can be used to make parts of PNG images transparent or semi-transparent. However, many editing tools seem to save alpha channel in PNGs by default, even if the image does not have any transparent areas. Whatever application you use to edit the images might have the option to remove alpha channel or save without transparencies. I usually get rid of the alpha channel with ImageMagick convert after creating/editing the files:
The alpha channel problem with the 240x320 screenshots and Asha banner has also been discussed (well, at least reported) earlier in this thread:
convert.exe oldfilewithalpha.png -background #00ffff -flatten -alpha off newfilewithnoalpha.png
Btw, the 240x320 screenshots are not really required, even though the website says so. The only new thing required for Asha 501 is the 120x60 pixel banner.