2D Platformer collisions

I'm working on a side-view 2D platformer and I'm having trouble getting collisions between the player and the level to work. Reading through both BlitzBasic and BlitzCoder, there are plenty of examples of 2D platform code that can easily be ported but don't help. The player is a PNG file and so is the level, and I can't get them to collide properly. Here's the source I'm working with (that actually works to an extent):

While Not KeyDown(KEY_ESCAPE)
	
	Cls
	ResetCollisions
	Update()

	DrawImage(lyr3,lx,ly,0)

	DrawImage(lyr2,lx,ly,0)
	DrawImage(plyr1,lx+X,ly+Y)

	DrawImage(lyr1,lx,ly)
	
	Flip
	FlushMem

Wend

EndGraphics()
End


Function Update()

	If KeyDown(KEY_D)
		Xvel = 2		
	ElseIf KeyDown(KEY_A)
		Xvel = -2
	Else
		Xvel = 0
	EndIf

	If KeyHit(KEY_F) Then Falling = True
	
	If ImagesCollide(plyr1,X,Y+1,0,lyr2,lx,ly,0) = False
		Falling = True
		FallSpeed:+accel
		Yvel:+FallSpeed
	Else	
		Yvel = 0
		Falling = False
		FallSpeed = grav
	EndIf
	
	If KeyHit(KEY_SPACE) And Yvel = 0
		Yvel = -18	
	EndIf
	
	X:+Xvel
	Y:+Yvel

End Function

The first layer is the foreground drawn on top of everything, the third is the background drawn first. The second layer is a simple image comprised only of pure red pixels and some heavily dark yellow pixels (for ground), simple so there's no messy problems occuring in collision checks. The player is an alpha-blended 32-bit PNG.

How can I do the player collisions properly? (btw this is all temporary code, reason why I don't have object methods and functions)