Is it okay to use pictures from Wikimedia Commons to help me map?

TL;DR (… but you should): I think that extracting info like building level count, height estimates from someone else's photos is okay to be used in OSM without a restriction, unless you take this photo from a georeferenced collection of photos like Google StreetView. As far as I know this is no easy topic.

I will try to give an overview from only a legal POV, and of course IANAL:

• All fully public domain data/works ("public domain", "CC0", WTFPL, …) should be no problem to use.
• More restrictive licenses there are harder. First, is the information you are going to derive from the "pictures" protected somehow and, second, is the license of the protected data compatible with the CT? See https://wiki.openstreetmap.org/wiki/Open_Database_License/Contributor_Terms/Open_Issues . Somehow this is discussed too at can-i-use-google-streetview-to-help-create-maps.
  • If we assume that the extracted information is protected then I guess only "pictures" under a non-share-alike license have a chance to be used for OSM because the share alike would mean that you need to publish the full OSM data under the same license which you cannot. For those licenses which only have an attribution requirement the question is: can we fulfil this requirement. Some people seem to think that we can fulfil this requirement by listing the sources on https://wiki.openstreetmap.org/wiki/Contributors (you find many CC-BY sources there) plus possibly in the changeset comments. In fact, I personally doubt this really is a sufficient attribution for several reasons. See https://wiki.openstreetmap.org/wiki/Open_Database_License/Contributor_Terms/Open_Issues#Incompatibility_with_CC-BY_.2F_Attribution_Data and can-i-use-or-trace-from-cc-by-data-under-the-new-contributor-terms regarding CC-BY sources.
  • If we assume that the extracted information is not protected then you could just use them. I think that if you extract information like "how many levels does this pictured (on a photo) building have?" then this fact is usually … not protected by copyright (even if it would be from a text instead of a photo). Maybe unless a user or a group of users collected photos to create a building height database … okay, maybe they did by grouping their photos into the Wikimedia Commons categories like "buildings by height"? Then we could think of this as a database – with no free license. Be aware that geocoding information on Wikipedia articles or Wikimedia Commons pictures might be derived from sources like google maps (instead of a own GPS measurement). Yes, I am aware that this thoughts may be exaggerated.

For all sources, attribution is a nice thing to do. … but obviously not on the "Attribution" wiki page which would get very very long then.

For all sources, a bit care is useful because the displayed license could simply be inserted by another wiki user, the uploader could not be the author (meaning that the uploader is not entitled to issue a license for the work). One case would be e.g. photos of maps (which were made by other people).

Maybe @SimonPoole from the LWG can shed more light here (see https://wiki.openstreetmap.org/wiki/Legal_FAQ#2b._XYZ_Organisation_has_data_for_free_download_under_licence_N._Can_I_use_it_in_OSM.3F which is about data – not photos – use).

From a data quality POV the there would be several other questions which should not be a part of this question here, I just wanted to mention it. Mentioning the source of your data is good anyway to enable other mappers to verify and understand your changes.