The display shows that pattern when it is first powered up and has not been configured by the library using the control and data lines.
The good news is the you have the display powered correctly and the backlight working. There will be 6 more connections to data pins that you need to get exactly right as shown in this example here. Some pins will not be suitable for this (pins 0 and 1 on an UNO as they are used for the serial port).
You also need to setup the right pins in the sketch, see example here
, otherwise the display will not get configured. It is critical that the pin order is correct.
Check solder joints are OK, no shorts between pins and nice clean wetted joints.
It looks like you are trying to use a single resistor, not a pot. as in diagram above to set the contrast. You should use a pot, but that is your next problem when you have the interface working.