You have online tools for it but standalone is better take paint.net its good enough for most people. There many more software that is free just google on photoshop free alternatives you will find a few. Photoshop is not that expensive any more in the US you pay its a Photography package that includes Photoshop and Lightroom its $9,99 a month. The best is that Photoshop has lots of tutorials on youtube.
I've just installed the most recent SnagIt release.
It's basically a screencap application with a ****load of extra features such as adding stamps.
It could definitely fit the request of creating HD images for blog posts. If you decide to purchase - you can *kinda* get away with using the education pricing for a wee bit of a discount: $29.95
It's a great little application that I've sworn by for years now.