How and where should images be cited? I am currently working on a paper where we use one image from another paper. How and where should I denote that?

Currently, we have the image source only in the caption like this:

\caption{Comparison of a \gls{CNN} for classification (top)
         and a \gls{FCN} which creates a heat map (bottom).
         \cite{long2014fully}}
The right thing to do when plan to include an image from another article is to first ask for the permission from the corresponding author of the article. If approved, you may include the image and cite them along with the lines after the caption


  Included with permission from [citeno].


General citation would only allow you to refer information in literature. Any form of replication whether it is an image or even a set of sentences of text is not permitted without acknowledgement from the authors of the published material to be referred. The exception is for quotes and equations (with appropriate citations). That doesn't forbid you to use the text at all. It means that you ought to rephrase the information referred in you own words.