Setalpha by distance traveled

Here's one based on your code above, but changed a bit in places.

Rather than set a dx,dy using Ints, here we pass in a random direction (0-359), as well as a random speed. The random speed thing gives you a bigger "spread" of particles.

It now runs on a max number of frames, rather than distance, which it simply counts down, removing the particle when the count is 0.
Fading is based on a range of 1 -> 1/num frames.

SeedRnd MilliSecs()

Global ParticalList:TList = CreateList()


Type TPartical
	Field x:Float,y:Float,a:Float = 1,r:Int
	Field ox:Int,oy:Int
	Field img:TImage
	Field dx:Float,dy:Float, da:Float
	Field maxdistance:Int
	Field fade:Int
	
	Method Create:TPartical(_x:Int, _y:Int, _img:TImage, _dir:Int = 0, _speed:Float = 0, frames:Int = 0, _fade:Int = False)
		ListAddLast(ParticalList,Self)
		x = _x
		y = _y
		ox = _x
		oy = _y
		img = _img
		dx = Sin(_dir) * _speed
		dy = Cos(_dir) * _speed
		da = 1.0 / frames
		maxdistance = frames
		
'		If mxdist
'			maxdistance = mxdist
'		End If
		If _fade
			fade = True
		End If
	End Method
	
	Method Update:TPartical()
		x:+dx
		y:+dy
		If fade a:-da
		SetRotation(r)
		SetAlpha(a)
		DrawImage img,x,y
		SetRotation(0)
		SetAlpha(1)
		
		maxdistance:- 1
		If Not maxdistance Then
			Destroy()
		End If
		
		'If fade
		'	a = DistanceTraveled()/maxdistance
		'End If
	End Method
	
	Method SetPAlpha(alpha:Float)
		a = alpha
	End Method
	
	Method SetPRotation(rot:Int)
		r = rot
	End Method
	
	Method Destroy()
		ParticalList.Remove(Self)
	End Method
	
	Method SetX(_x:Int)
		x = _x
	End Method
	
	Method SetY(_y:Int)
		y = _y
	End Method
	
	Method DistanceTraveled:Float()
		Local returnx:Float=Abs(ox-y)
		Local returny:Float=Abs(oy-y)
		Return returnx+returny	
	End Method
	
	
End Type


'Partical explosion function; creates a explosion of particals
Function ParticalExplosion(_x:Int, _y:Int, image:TImage, n:Int, frames:Int)
	
	For Local i:Int = 1 To n
		Local dir:Int = Rand(0, 359)
		Local speed:Float = Rnd(0.08, 1.5)
		Local part:TPartical = New TPartical.Create(_x, _y, image, dir, speed, frames - 10 + Rand(20), True)
	Next
	
End Function

AppTitle = "Partical test - click to fire"
Graphics 800,600,0
SetBlend alphablend

'Create a dot image (our partical)
Global dot:TImage = CreateImage(5, 5) 
DrawOval 0, 0, 5, 5
GrabImage(dot,0,0)

While Not KeyHit(KEY_ESCAPE)
Cls
	For Local UpdateParticals:TPartical = EachIn ParticalList
		UpdateParticals.Update()
	Next
	If MouseHit(1) ParticalExplosion(MouseX(), MouseY(), dot, 50, 100)
Flip
Wend
End