There several important findings here. First, note that there is a destructive process involving both sides of a joint. Second, the subchondral line on both sides of the first MTP joint is very ill-defined and indistinct (look at the second MTP for comparison). Third, there is a pathological fracture of the distal first metatarsal. Fourth, there is a moderate amount of periosteal new bone formation along the metatarsal shaft.
The first and second findings are strong evidence that we are dealing with a joint infection. Tumors could cause similar bone destruction and periosteal reaction, but usually don't involve two adjacent bones. Various other arthropathies could cause marked erosions and joint destruction, but usually leave part of the subchondral line intact.
The subchondral line is seen along both sides of just about every joint in the body, as well as along every vertebral endplate. Loss of the subchondral line should certainly raise the question of joint infection. However, this finding is not specific for infection. It just tells us that there is probably some type of joint inflammation present. For example, the subchondral line in the sacroiliac joints is often lost in ankylosing spondylitis.