std::string + const char* results in another std::string. system does not take a std::string, and you cannot concatenate char*'s with the + operator. If you want to use the code this way you will need:
std::string name = "john";
std::string tmp =
"quickscan.exe resolution 300 selectscanner jpg showui showprogress filename '" +
name + ".jpg'";
system(tmp.c_str());