Hello,
Before I can download images from my camera, avast seems to do a complete scan
of it’s content. As my flash memory can contain hundreds of photos, and as I do
theorically need to check only my last photos, here is any mean to tell avast not to
scan my camera, or to stop it ?
Thanks in advance for your answer !
If you are not connected to the internet while downloading images from your camera, you could stop on-access protection until you finish. I think this would do it and if I am wrong, someone please correct me. 
Please do not forget to reverse this before you re-connect to the internet.
Add the path to the Exclusion Lists.
For the Standard Shield provider (on-access scanning):
Left click the ‘a’ blue icon, click on the provider icon at left and then Customize.
Go to Advanced tab and click on Add button…
For the other providers (on-demand scanning such as the screen-saver or the Simple User Interface):
Right click the ‘a’ blue icon, click Program Settings.
Go to Exclusions tab and click on Add button…
You can use wildcards like * and ?.
What is the sensitivity of the Standard Shield ?
Mine is set at Normal (the default) and I have just tested downloading some images from my camera and it doesn’t appear to scan them all. My photos are stored as jpg format so I don’t know if your images are the same or the sensitivity is set to High.