Help on improving the query performance of MySQL table

My team handles a tool that automatically detects and categorizes images.We have a Mysql DB (InnoDB engine) used by our tool in production where we store information about each image processed.

The table is poorly designed by someone long before I joined the team. It was all working well as there was very less data in Db till now. Recently we launched the tool to a wider audience and the Db is now having huge data. The ‘select’ query speed is very slow and takes days to get result.

I am not an expert in Databases. Please help on suggestions to improve the performance. I am thinking of options like creating indexes. We cannot have downtime for the system.Th Db is write heavy (around 200 insertion per second). I am afraid of creating table locks if i try out index creation.

Table schema: 6 columns , all Varchar, no primary Key, no indexes