The simple Microsoft Photodraw for Windows Version 2 will do all you need, Steve.
This was often included in 'Office' and was not very sophisticated, so any [probably 'Freeware' ] image manipulation programme will do it too.
You will need 'Crop', 'Resize', 'Cut', & 'Paste' controls at least, and 'Erase' would be useful too.
The basic stages are
1 Take your photo or find it. Use the highest quality setting on your digital camera.
2 Display the picture [Doc A]in your chosen programme, enlarge it so that the bit you want [eg a door]nearly fills the screen and 'crop' it. If indeed it is a door, you will be able to get very close to the vertical and horizontal edges you wish to keep. You can also erase little bits that you don't want to include.
3 Paste it to a new document in the same programme.[Doc B]
4 Resize it in Doc B using the programme to suit your scale, i.e., if it is a front door about 6ft-6" high, multiply 4mm by 6.5 = 26mm for OO, etc.
5 Cut and paste re-sized dioor into another document in the programme [Doc C] so that if Doc C were to be printed out on a sheet of A4 paper, there would be a little 26mm high door on it. If you want more identical doors, paste it as many times as you need! Repeat the process with as many other building elements as you need, until you have a collection of bits and pieces that you can choose from to complete a building project, available to print, cut out and glue to appropriate pieces of card to be added to your model.
Scaling the elements is the biggest challenge, remember for a rough guide, a brick is 9" long [0.75ft] so you could work out some dimensions that way. If I'm out with my wife who is 5'-4" tall and wish to photograph a particularly interesting building, I stand her right in front of it, cross the street and take the picture. Then in Doc B I estimate how many times 64" go into the height of the building, for example, and work from there.
I hope that helps, first get your programme and have a play wih it!