This blog post is to demonstrate the optimum settings for a blog post image that is really about the text, so that when the post is shared on facebook the picture comes out in full. Based on only having one image in the blog post and just sharing the link to the blog post and accepting the default image selected by facebook.
I read on theloneliestplanet.com that the dimensions of an image need to be 560 x 292 pixels. I experimented with making a text image using PowerPoint and found that doubling pixels, but sticking to the proportions worked best.
I have my PowerPoint set to an export resolution of 300dpi instead of the 100dpi default. To change the settings for the version of PowerPoint you have see: http://support.microsoft.com/kb/827745
For the above image, I changed the slide master page setup to custom dimension page size W 9.49cm x H 4.95cm (landscape), which gives an image W 1120 x H 584 px. Saving as a png rather than a jpeg gives crisper letters for a text based slide.
Please share this blog post on facebook to see how the image is displayed for you and your followers.