Can I tighten the drive belt?

I see no way to adjust the pulleys on my 2007 48 inch DYS 4500 917.287301 mower. I've boughten two belts at different times and each time part number 130969 a 42 inch belt measures 43 inches and fits just loose enough it slips so the mower has trouble moving on inclines and eventually won't move.

We had a guy look at the mower when we couldn't identify the problem and I could of swore he said he tightened it to fix it. I sure don't see how looking at these pulleys. All three look fixed to me and I've lost track the guy I'm kind of stuck. Is it normal for the belts no to be the size they say? If it was 42 inches it would be perfect. Does anyone know specifically about this mower system? Please help. :)

Last resort I suppose I'll try to find an actual 42 inch belt . lol