Adding the description is easy if you use the sample screen (remember to set BOBCACHIEVEMENT_SCREEN_NAME):

screen bobcachievement_samplescreen(achievement_title, achievement_description):
    timer 5.0 action Hide("bobcachievement_samplescreen", transition=BOBCACHIEVEMENT_SCREEN_TRANSITION)
    vbox:
        xanchor 1.0 xpos 0.95 yanchor 0.0 ypos 0.05
        style_prefix "bobcachievement"
        text BOBCACHIEVEMENT_NOTIFY_PREFIX text_align 1.0 xalign 1.0
        text achievement_title text_align 1.0 xalign 1.0
        text achievement_description text_align 1.0 xalign 1.0

Unfortunately, adding an image is going to be harder because I didn't pass the achievement names to the screen, so you'd have to update that:

Change line 75 to

renpy.show_screen(BOBCACHIEVEMENT_SCREEN_NAME, achiname, BOBCACHIEVEMENTS_MAP[achiname][0], BOBCACHIEVEMENTS_MAP[achiname][1])

And then this screen (or something like it) should work:

screen bobcachievement_samplescreen(achiname, achievement_title, achievement_description):
    timer 5.0 action Hide("bobcachievement_samplescreen", transition=BOBCACHIEVEMENT_SCREEN_TRANSITION)
    hbox:
        xanchor 1.0 xpos 0.95 yanchor 0.0 ypos 0.05
        add "images/achievements/" + achiname + ".png"
        vbox:
            style_prefix "bobcachievement"
            text BOBCACHIEVEMENT_NOTIFY_PREFIX text_align 1.0 xalign 1.0
            text achievement_title text_align 1.0 xalign 1.0
            text achievement_description text_align 1.0 xalign 1.0