Congratulations first, it looks good ! just a a shame it wasn't a proper agreed paid assignment.
As others have said, 1 they have at very least taken the professional piss, as they have presumably paid a designer, ad space and probably a marketing / ad company to be involved in the production.. so they really should pay for the photography , and they will know this very well!
2 as Unfocused said work produced when working for your employer is property of the employer.. probably partly true but what clause in your contract says any photography or other work not electrical as specified in the job sheet that week/ month is their property? I doubt it , gut feeling says if you took this to court it would go in your favour.
As a note if you work as a staff photographer or designer then your contract will probably say that any images/work produced while on time / job / assigment are their copyright but everyone knows this beforehand. Seems to me your case is very different.
As you were doing a favour to the company, and all you were aware that they would be used in newsletters and in-house / union PR or maybe an exhibition and you say you are fine with that, most other people in a similar position would be too.
However and a big however, as this is now Advertising material, not just some pics to show the rest of company some ongoing work or best for an inhouse/ industry only union PR event. In my work internal company PR/ news vs advertising are very different uses ( and fees)
PR and company news updates etc will often use a fellow employee who is keen amature to take the pics. As they don't have much monetary commercial value then thats fine, and photog involved enjoys some low level limelight, and company enjoys his hobby and other skills which make them a bit more valuable or liked.
This is much different, presumably they have paid for ad space, a designer and ad company to produce it - in which case your picture was used without consent for this use and normally they would have to pay somewhere from $400-$1000 to get a shot like this from professional photographer.
But I would first check if it was all produced in house by your company - was it designed by in house designer / marketing staff? and they they organised the ad space.
If it was a kinda off the cuff idea from some of the staff and they did it all theirself, especially if they don't normally do this type of thing but thought the pic was great so lets do this. Then it might be unintentional and innocent. which case you should still have a 'nice' chat with them and see if they would contribute to your equipment in future ( state how much they have saved using your picture so say 50% of what they might have paid would be a good gesture.
But if it was made , organised or planned as part of an advertising drive then you really should be compensated paid for the usage of the image. BTW outdoor/ billboard / poster advertising normally attracts a 50%-100% fee on top of the photography shoot fee. SO use this to add validity to your case, at least you can say to help you pay for some new equipment to help take better pictures next time - but next time agree on the usage, make sure its only for newsletter or website blog or maybe a union exhibition but not for advertising / marketing use.
Probably best write them a nice email stating how and where any pictures you supply in future. just so you all understand, as for ad / marketing even PR to get as good shots as you can supply they would normally have to pay fairly decent money.
You may just have to take this one on the chin, if so I'd raise in email how you liked how they appreciated your picture enough to run it on an ad but aren't totally happy with this unsolicited use this time, and but will let it go for this one billboard.. however any more they would like to run it on they will have to come to some agreement about further usage and $.
In the future please do clarify the usage for anything else you do... hopefully they will appreciate your work and offer some $$ for you next time
Additionally you are in a very good position offer them really good photography, as you have the skills, plus inside knowledge about what & how should be shown in the images and be able to effortlessly get the best out of any staff you know. These things are as important or more than just good photo skills.