How can i thoroughly analyse a source?

I always find it useful to cover up the actual image/picture initially, and focus first on the caption and what that can tell you about the souce - this also works to make sure the context and provenance are considered in reference to the question. 

The next key thing to do is to consider how much the source can tell you about whatever topic you are focussing on - however, it is important here not just to describe the source, but to attempt to make simple inferences on the knowledge that can be drawn from the source. If there are any gaps in the source you should attempt to fill them with your knowledge, not just leave them blank!

The last thing to consider should be the reliability of the source in terms of the question - what is the purpose of the source? This is particularly important in the case of sources such as commissioned paintings/pictures and magazine/journal/newspaper articles as there is a high potential for bias! When assessing the reliability of a source, it is also important to look at the context and work out if there are any contextual mishaps which may affect the conclusions you reach. 

As with any kind of analysis, the key element to remember is to ALWAYS link back to the question and ultimately to keep in mind what it is you have been asked to look for, not just to state everything you can see.