Saskia,
Once you click on the + (plus sign) in the blue menu bar of Collector you are taken to the collect screen. Once you select a features symbology which you designed in the original service that you uploaded a new window opens with 4 items in the blue menu bar. They are in order from L to R Cancel, a gear symbol, a stylized map symbol, a camera and the text Submit.
Click on the camera icon and a new window opens entitled Attachments. Click on the ADD button in the window and you get a choice to Take a Photo or Video or Choose from Library. If you choose take a photo, the camera app opens and you take a photo as normal. Confirm, Use Photo and photo is added to the attachment screen. You can take multiple photos at a given location, simply tap the Add button again.
Once done, in the upper left of the attachment window is the text DONE in very light blue that is hard to see.
This take you back to your main screen and if all data is correct hit the SUBMIT text in the upper right to update the AGOL data.
If you want the images associated with the collected features for use in ArcMap, you need to export the AGOL data as a file geodatabase (FGDB). Exporting as a shapefile does not attach the images.
As mentioned previously you need to have established attachments when the original FGDB was created for uploading to AGOL and use in Collector.
Hope this helps.
Dave
