Replace JPEG graphics with PNG graphics that have transparent backgrounds

[jakepetroules]

This relates to axr/core#151 as many of these graphics were created using HSS. We should provide the HSS sources for these graphics in the folder next to the PNGs, which can be automatically generated using the axr2img tool in the extras repository.

Here is a list of pages that contain graphics which need updating. Please keep this up to date.

• http://axrproject.org/blog/2011/hello-world
• http://axrproject.org/blog/2012/what-has-happened-in-the-axr-project
• http://axrproject.org/blog/2012/new-release-available-version-0.4.8
• the 404 page graphic

[Ragnis]

I'm just saying, but the "the 404 page graphic" belongs to the axr/website
repository since the 404 page is not in this repository.