how do I make 300px PNG file size smaller

I have some images being output with phpthumbof 1.4.0 all is working great with Revo 2.2.10
The images are built into a slider and I need them to be 300px in size, what would be nice is to add rounded corners but I need the background transparent or colored to stop the ugly white edges left behind on the rounded edges.
If I switch to PNG images it works but the file size goes from around 30kb with JPG to about 280kb with PNG.
My question is can I make the file size of the PNG images smaller I cant find any settings that allow this.

Thanks

Ian

Just render your pictures as jpg and round them using css border-radius.
Fallback for IE 7/8

OH yes of course overlooked the most obvious answer DUH!!!

Thanks for the link very useful have bookmarked that.

Cheers

Ian