You can store images only in 1 folder BUT, for example you have device that is mdpi.. it will look good on him, but if you run your app on ldpi android will automatically scale your image and it will look ugly (low quality). So yea you need different resolution based images for different types of devices.. so mdpi resolution images go to drawable-mdpi so hdpi resolution images go to drawable-hdpi etcc..
this is some list i found on internet about screen sizes, maybe you will find it helpful:
Low density Small screens QVGA 240x320 (120dpi):
layout-small-ldpi (240x320)
layout-small-land-ldpi (320x240)
Low density Normal screens WVGA400 240x400 (x432) (120dpi):
layout-ldpi (240 x 400 )
layout-land-ldpi (400 x 240 )
Medium density Normal screens HVGA 320x480 (160dpi):
layout-mdpi (320 x 480 )
layout-land-mdpi (480 x 320 )
Medium density Large screens HVGA 320x480 (160dpi):
layout-large-mdpi (320 x 480 )
layout-large-land-mdpi (480 x 320)
Galaxy Tab ( 240 dpi ):
layout-large (600 x 1024)
layout-large-land (1024 x 600)
High density Normal screens WVGA800 480x800 (x854) (240 dpi):
layout-hdpi (480 x 800)
layout-land-hdpi (800 x 480)
Xoom (medium density large but 1280x800 res) (160 dpi):
layout-xlarge (800 x 1280)
layout-xlarge-land (1280 x 800)
Also it would be good to read official documents site about supporting different types of screen.