This question comes up quite a lot so I thought It was time for a quick article about it. 
Since the change to 2.2, a new permission level has been created, the "Download" permission. This will need to be checked to allow group(s) to download (or in the case of images view) them.
Check this setting (for each forum you wish to allow) > ACP > Management > Manage Forums > (edit each forum as needed) > Permissions
Next, lets check the allowed amount of space, both per post and in total. This is adjusted per member group. Note that this is per member, not a group total.
ACP > Management > Users And Groups > Manage User Groups > (edit each group as needed)
Note the sizes are in KB. In my example each member of that group has a total attachment space of 1024KB (1MB), and a per post limit of 128KB (this can be one or several attachments up to 128KB per each post)
(To get a figure in KB, simply multiply it by 1024) , example:
128KB = 0.1MB
512KB = 0.5MB
1024KB = 1MB
2048KB = 2MB
5120KB = 5MB
10240KB = 10MB
25600KB = 25MB
51200KB = 50MB
102400KB = 100MB
Since the change to 2.2, a new permission level has been created, the "Download" permission. This will need to be checked to allow group(s) to download (or in the case of images view) them.
Check this setting (for each forum you wish to allow) > ACP > Management > Manage Forums > (edit each forum as needed) > Permissions
Next, lets check the allowed amount of space, both per post and in total. This is adjusted per member group. Note that this is per member, not a group total.
ACP > Management > Users And Groups > Manage User Groups > (edit each group as needed)
Note the sizes are in KB. In my example each member of that group has a total attachment space of 1024KB (1MB), and a per post limit of 128KB (this can be one or several attachments up to 128KB per each post)
(To get a figure in KB, simply multiply it by 1024) , example:
128KB = 0.1MB
512KB = 0.5MB
1024KB = 1MB
2048KB = 2MB
5120KB = 5MB
10240KB = 10MB
25600KB = 25MB
51200KB = 50MB
102400KB = 100MB