Merge two pictures could be real fun and produce a great effect while viewing, the over all transparent and overlay effect make both the images visible to some extent and both the image got merged together in a beautiful way.
But, why would any one want to merge two images? it could be for fun and also for merging two different type of maps together.
There are two free program which can merge images together, first one is called S_Merge which is very easy to use program unless you follow the clear instructions provided by the program interface itself.
It also lets you preview the final merged image and you can also save a cropped portion of the final merged image you obtain.
The second software which lets you merge images together is STG Picture Merge which is again more easy to use as compared to S_Merge.
You just need to load the first and second image one by one to obtain the final merged images, moreover you can preview the image before saving and can also control the transparency effect.