Where to include acknoledgement for used graphics In the current technical report that I write, I have used graphic from an online-source where their policy requires that the author of the drawing has to be acknowledged in case that the drawing is to be used. 

This policy applies to electronic and printed formats of the given work.

Where do I place the acknowledgement for this drawings? Probably the proper way would be to use footnote in the page where the drawing is shown but, the policy requires that the name of the author is written. I would like to avoid such content in the footnotes where I discuss important information.

Is it OK if I do the acknowledgement in an appendix in the end of the report?
You want the reference close to the actual picture. You could go with @Memj's suggestion in the other comment. Or simply put it in the caption:

Fig. 19: Measured relationship between galaxy distance and
red shift. (Figure reproduced with permission from http://abc.org,
copyright Adam Smith, 2015.)