Hi
Helena - I'm assuming that you want to view your camera photos that are on a SD card (whether inserted into your computer or attached via the digital camera) and delete those from the camera before transferring to the computer - correct?
If so, one option is to go into Finder and select the SD card which will be mounted; then, choose the images you want to view (i.e. click the ones of interest or draw a box around a bunch of pics) - double-click on one image and all should show up in a Preview window (see attached pic below of a recent trip for me); on the right, swiping will allow you to pan through the chosen images; on the left, click on an 'unwanted' image, then 'double click' and select move to trash - that image(s) will be deleted from your SD card - give it a try and let us know if the process works for you - good luck! Dave