Your photo is fine.
It is an ISO-400PP psu which is a 300w standard ATX unit as suspected.
It is not clear to me what your issue might be with USB3.
There should be no psu cables that have anything to do with usb ports.
Likely, your case will not have USB3 front ports that your new motherboard supports.
If you want usb 3.0 support, there are several ways to go about it.
If your case has a 5.25" front bay available, like many old ones did, they you can install a front panel with all sorts of components including usb3.
Your new motherboard will have usb 3 ports on the rear I/O panel so you could use one of them.
Regardless, I would support buying a newer psu.
A quality psu is something you should not go cheap on.
While 300w might be sufficient for now, a good quality stronger 450w or 550w psu might be a good investment.
A psu will only use the power demanded of it, regardless of the max capability.
Replacing a psu is not difficult.
Cables are keyed to fit only one way. There is a series of rounded and square pins.
So long as you do not force anything, you will be ok.
Expect to pay $75-$100 for a quality unit.
Here is one list of psu quality tiers:
Credit to @Starelementpokeand @Energycore for helping out with this long-awaited refresh of the old PSU tier list by Aniallation (who previously went several months without logging in), leaving the old list in need of help. This is simply an update of the list that was flawed in creation by the P...
linustechtips.com
Look for tier 3 or better.
If in doubt, Seasonic is always decent.