Yeah, those pictures are 'odd' as you don't often see Asian in German looking uniforms. There's even a fairly well known picture of Generalissmo Chang's (Chinese leader at the time) son in full German Wehrmact uniform. LIke many military officers at the time, he was trained in Munich and even earned his Edelweiss wit the elite German mountain troops. Eventually he commanded a tank but was recalled to China before Poland was invaded.
In any case, back to you question, the Germans had a miliary mission in China was decades. They supplied training and sold tons of armrament to the Nationalist goverment in the late 20's and early 30's. The Chinese had purchased German Heinkels, panzers, and halftracks. I have attached some of those pictures. The mission ended in the late 1930's after pressure from Japan caused the recall of German advisors.
After the recall, the Russians became the major supplier of arms to China. Not only did Russian supply a large amount of aricraft, they also had Russian pilot 'advisors' who fought against the Japanese in Russian built Chinese airforce aircraft.
The Russians withdrew most of their military support in 1940 after they needed every resource available for the worsening situtation in Europe.
The Americans came in 1940 starting with the Flying Tigers, and then became the major supplier of weapons to the Chinese via the Lend lease act after Pearl harbor. Here's some pict's of American armor operated by the Chinese troops in Burma.