From Flickr users, under creative commons:

• 'all posts' header image - 'Puppeteers flee the exploding galactic core' by RedToby
• 'behind the image' header image - 'blur, walking through the fallen leaves lomotic ver' by itchys
• 'consulting' header image - 'up go the lights' by mccun934
• 'about Andrew' header image - 'Octopod' by quinet 
• 'following Christ' header image - 'vignette_fractalius' by Johanne Brunet

From the public domain:

• Space images by NASA
• Various blog post images

All other images by Andrew Mackie.