On many Citroen car models the alternator can be removed without touching the power steering pump. However I don't have a manual to hand. Because the Picasso is a car derived mini-people mover with the 'cab forward' body shape, the engine is harder to work on. If you haven't got a Haynes manual then it will be money well spent to buy one. I'm guessing you will have to remove engine under tray and side trays to get to expose the auxillary belt or belts. Normally these ribbed non synchronous belts are tensioned quite taut by manually or automatic tensioner. To release the tension so you can remove belt and thus the alternator, you either have to move the tensioner using a spanner or hex/allen key (depending on type). Some Citroens use a threaded rod/long bolt and a bracket to release alternator itself. Again my instructions are generic to most makes of car.
You may have to raise front of vehicle. Jack it up or use ramps but whatever method you use, chock wheels and support car on axle stands before getting under it. Safety first. Don't work on a car when tired or under the affluence of incahol.... :?
You should disconnect battery before working on engine.
Make sure you make note of any radio codes beforehand.
After disconnecting battery, unplug the alternator. Make a note of what goes where.
Before you remove the belt, make a sketch of the belts routing around the crank pulleys, tensioners, guides, alternator, air con pulley and power steering pump. Long belts are quite confusing to refit if you havent got a diagram to work to.
Once belt is off, check its condition. Any cracking, chunks missing, excess polished look to smooth surface, oil contamination, etc means a new belt. I've seen snapped alternator/auxillary belts snap and tear into the timing belt cover causing major damage...
Anyway, if you've got the belt off, have a feel and rotate the pulleys, guides, alternator pulley and air-con & power steering pump pulleys. If any feel loose, rough or make 'orrible grating noises or feel hard to spin, then it means wear or partial seizure. Having said that, a power steering pump will have some resistance to turn by hand because after all it is a pump.
Assuming all your guides and tensioners are ok, then have a look at what is holding the alternator on. Normally these are fixed using two or more bolts - one being a long pivot bolt and maybe an upper nut and bolt and/or threaded adjuster. Again, it depends on the car.
You may have to remove a bolt on strengthening bar that supports inner wing/chassis rail to subframe. Larger Citroens often have these as did my old XM. If the clearance looks tight, have a look at power steering pump fixings. If there's no sign that it's adjustable then chances are it doesn't have to be touched.
These are just a general guidelines common to many cars. Apologies in advance if your is different as I've not worked on many Picassos and the last one I did work on was in for general service.
If you could email me a picture of the engine or put a pic on this forum I could see what needs to be removed or at least point you in the right direction.
Let us know how you get on